Question

Arrange the following groups in order of increasing priority. \(\begin{array}{lll}\text { (a) }-\mathrm{CH}_{3} & -\mathrm{H} & -\mathrm{Br}\end{array}\) \(\begin{array}{llll}\text { (b) }-\mathrm{OCH}_{3} & -\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} & -\mathrm{B}\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right)_{2} & -\mathrm{H} \\\ \text { (c) }-\mathrm{CH}_{3} & -\mathrm{CH}_{2} \mathrm{OH} & -\mathrm{CH}_{2} \mathrm{NH}_{2} & -\mathrm{CH}_{2} \mathrm{Br}\end{array}\)

Short answer

Based on the Cahn-Ingold-Prelog priority rules, arrange the following groups in order of increasing priority for each set: a) -H, -CH3, -Br, -CH2CH3 Answer: -H < -CH3 < -CH2CH3 < -Br b) -OCH3, -CH(CH3)2, -B(CH2CH3)2, -H Answer: -H < -B(CH2CH3)2 < -CH(CH3)2 < -OCH3 c) -CH3, -CH2OH, -CH2NH2, -CH2Br Answer: -CH3 < -CH2NH2 < -CH2OH < -CH2Br

Step-by-step solution

Arrange Groups in Group (a) based on CIP Rules

First, let's arrange groups in group (a): \(-\mathrm{CH}_{3}-\mathrm{H} \quad-\mathrm{Br} \quad-\mathrm{CH}_{2}\mathrm{CH}_{3}\) Compare the first atoms in each group: C(6), H(1), Br(35), and C(6). The order will be H < C < Br. So, now we need to figure out the order between the two carbon-containing groups. Since both of these groups start with a carbon atom, we need to look at the atoms bonded to these carbons. For the methyl group (\(-\mathrm{CH}_{3}\)), there are three hydrogen atoms, while for the ethyl group (\(-\mathrm{CH}_{2}\mathrm{CH}_{3}\)), there are two hydrogen atoms and one carbon atom. Comparing their atomic numbers, we have H(1) < C(6). Since the ethyl group has a higher priority atom (C) bonded to it, the order of priority will be: \(-\mathrm{H} \quad < \quad -\mathrm{CH}_{3} \quad < \quad -\mathrm{CH}_{2}\mathrm{CH}_{3} \quad < \quad -\mathrm{Br}\)

Arrange Groups in Group (b) based on CIP Rules

Now, let's arrange groups in group (b): \(-\mathrm{OCH}_{3} \quad -\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} \quad -\mathrm{B}\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right)_{2} \quad -\mathrm{H}\) Compare the first atoms in each group: O(8), C(6), B(5), and H(1). The order will be H < B < C < O. So, now we need to figure out the order between the two carbon and boron-containing groups. For the isopropyl group (\(-\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\)), the first carbon is bonded to two methyl groups and one hydrogen. For the B-containing group, the boron is bonded to two ethyl groups. Comparing their atomic numbers: H(1) < C(6) and B(5) < C(6). Based on the priorities of atoms bonded to the first atom (C or B), the order will be: \(-\mathrm{H} \quad < \quad -\mathrm{B}\left(\mathrm{CH}_{2} \mathrm{CH}_{3}\right)_{2} \quad < \quad -\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} \quad < \quad -\mathrm{OCH}_{3}\)

Arrange Groups in Group (c) based on CIP Rules

Finally, let's arrange groups in group (c): \(-\mathrm{CH}_{3} \quad -\mathrm{CH}_{2} \mathrm{OH} \quad -\mathrm{CH}_{2} \mathrm{NH}_{2} \quad -\mathrm{CH}_{2} \mathrm{Br}\) In this case, all groups have a carbon atom as their first atom. We will need to look at the atoms bonded to these carbons to determine the priority. Methyl group has H(1), hydroxyethyl group has H(1) and O(8), aminoethyl group has H(1) and N(7), and bromoethyl group has H(1) and Br(35). Comparing their atomic numbers, we have H(1) < N(7) < O(8) < Br(35). Therefore, the order of priority will be: \(-\mathrm{CH}_{3} \quad < \quad -\mathrm{CH}_{2} \mathrm{NH}_{2} \quad < \quad -\mathrm{CH}_{2} \mathrm{OH} \quad < \quad -\mathrm{CH}_{2} \mathrm{Br}\)

Key concepts

Organic Chemistry

Organic chemistry is the study of the structure, properties, composition, reactions, and preparation of carbon-containing compounds, which include not only hydrocarbons but also compounds with any number of other elements, including hydrogen, nitrogen, oxygen, halogens, phosphorus, silicon, and sulfur. This branch of chemistry was traditionally limited to compounds produced by living organisms but has been broadened to include human-made substances such as plastics. The range of application of organic compounds is enormous and also includes, but is not limited to, pharmaceuticals, petrochemicals, food, explosives, paints, and cosmetics.

Understanding the structures of organic molecules is fundamental to mastering organic chemistry. Any such structure can be analyzed through several aspects such as the presence of functional groups, the connectivity of atoms, and the three-dimensional arrangement of atoms in space, which is crucial when considering reactivity and interactions of organic molecules.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s), regardless of the size of the molecule it is a part of. Examples of functional groups include hydroxyl groups (-OH), carboxylic acids (-COOH), amines (-NH2), and halides (such as -Cl, -Br).

In organic chemistry, the properties of a molecule can be largely deduced from the types and arrangements of functional groups present. Identifying functional groups is an essential skill for predicting reactivity and learning reaction mechanisms. Functional groups also play a critical role in the formation of molecules, as they can interact with each other to produce larger, more complex compounds.

Atomic Number Priority

Atomic number priority is a rule used in organic chemistry to compare the precedence of different atoms within molecules, especially when determining the configuration of chiral centers or resolving E/Z isomerism. According to the Cahn-Ingold-Prelog (CIP) priority rules, the priority of atoms or groups is determined by their atomic number; the higher the atomic number, the higher the priority.

When comparing groups in the context of stereochemistry or double bonds, the first atoms of each group are compared. If these are identical, the next set of atoms are compared, and so on, until a difference is found. In the provided example, bromine (Br) has a higher atomic number than carbon (C) or hydrogen (H), thus making it the group with the highest priority in group (a). Understanding and applying the atomic number priority rule is imperative in assigning the correct R/S configuration or Z/E configuration to a molecule, which can have profound implications on its properties and behavior.

R/S Configuration

The R/S configuration is a stereochemical notation to describe the configuration of a chiral carbon atom in an organic molecule. 'R' stands for rectus (Latin for 'right'), and 'S' stands for sinister (Latin for 'left'). This system is part of the CIP priority rules used to denote the spatial arrangement of groups attached to a chiral center.

To assign an R or S configuration to a chiral center, one must determine the priorities of the substituent groups based on atomic number. The atom or group with the highest atomic number receives the highest priority. Once priorities are assigned, the groups are oriented so that the group with the lowest priority (typically a hydrogen atom) is in the back. If the direction of decreasing priority numbers is clockwise, the configuration is R; if counterclockwise, it is S.

This R/S notation is crucial when describing the three-dimensional structures of molecules because different configurations (enantiomers) can have very different biological, medicinal, or chemical properties, despite having the same molecular formula. Understanding and correctly assigning R/S configurations can be essential for the study of pharmaceuticals and biological systems, where the wrong enantiomer of a molecule can have ineffective or even harmful effects.